Taught By

Loudon Stearns

Assistant Professor

Transcript

In a synthesizer after the sound leaves the oscillator it enters the filter section. Now in a simple synthesizer there'll be a single filter. But in a more complex synth you'll have many filters. Now, we've already talked about filters in general. We've talked them in EQ setting, and I said in a mixing board that a low pass filter isn't incredibly useful, because it reduces the high end so much, kind of removes the high end completely. In a synthesizer though, the low pass filter becomes the most important type of filter because these geometric wave forms are so bright and so harsh we have to remove the high end drastically. So, the low pass filter, 24 decibel low pass filter, becomes the most important type of filter. And if you just say, the filter, that's what you're talking about. Most synthesizer filter's actions are state variable, they can change their shape. You can switch them over to be high-pass filters or band pass filters. But the most common is definitely low pass. Now like the oscillator and the synthesizer and the filter can be modulated, and the most important modulatable thing in a filter is going to be the filter cut off. And you hear it all the time in a synthesizer patch where it starts bright and then gets dull, kind of a sweeping sound across this across it, or a wah-wah type of sound. Well that is the filter moving over time. In fact, I'd like to make a little analogy to the human voice. If we think of the human voice, we have an oscillator and a filter. Our oscillators, are the vocal folds, right? They create a steady tone. It's actually kind of like a square wave with a narrow pulse with, it's kind of on and then off for a while, then on and then off for awhile. We call it a, a pulse wave. And the mouth functions as your filter. And in fact, if you're thinking of kind of making a synthesizer patch, that's actually a really useful thing. Try to vocalize it and see what changes. Like if I have a patch that goes, wah-wah. Well, that's going to be the filter changing, not the osillator, right? I move my mouth but my vocal folds, I felt like they were at the same pitch the whole time. So we have this idea of oscillator filter all over the place. And when we have a moving filter we get these kind of wah sounds or the sense of going from bright to dull. There's another important part of a filter that you want to be aware of. The filters in a synthesizer tend to be resonant filters. In a mixing board, we kind of avoided boosts a little bit. We didn't want to really draw attention to the EQing, we're trying to keep the sound a little more pristine. But in a synthesizer anything goes. And often we do want to have a major boost in the sound of the filter becomes an important characteristic of the synthesizer itself. Some people would say the fill, the sound of the filter is the most important part of the synthesizer. And resonance is actually related to feedback on a delay line. We said that all filters are really delays. Well, delays have feedback and that's what resonance is. As you turn up the resonance it emphasizes the cutoff frequency, so instead of the low pass filter looking like this, it has a big peak right off that cut off frequency, and it can be very obvious when it's swept. In fact, the resonant peaks, it can be so obvious as it's swept if you have a really peaky resonance, if it's really high, you'll hear each harmonic kind of jump out as it sweeps across the the, the, the spectrum and across the note. So there are many things that can modulate the filter. Right? very commonly key position does. You want that filter to move up and down along with the key position. And you can also have envelopes and LFOs modulate the filter frequency. All you want to remember with this and the most useful thing with this is if you, if you pick a patch and it's too bright or too dull. Go to that big filter cutoff frequency knob. It's also a great one to modulate over time. It's very dramatic and it's a really important motion in electronic music. Let's look at the filter section of our simple synth before moving onto the amplifier section. We're now going to examine the filter section on this simple synthesizer. I'd like you to take a moment to go to your synth, and see if you can locate the filter section there. Mostly likely, it'll be called filter, or VCF, which stands for Voltage Controlled Filter. Or FIL, and most likely, you'll have options for how to adjust that filter, between the standard filter shapes of low pass, high pass, band pass, and band stop, which also might be called notch. What I'm going to do first is filter some noise. And this way we'll get to see what the different filter shapes are. But we've already covered this a bit in the course, it shouldn't be that difficult. We'll start with a low pass filter. [SOUND] And now I'm using the low pass filter, because it's a standard synth filter, and I'm manually modulating cutoff frequency. I'll start off with the high and we'll try a sawtooth waveform. Now a sawtooth waveform can be very obnoxious [SOUND] but if I start start lowering the cutoff frequency of the filter it becomes a much more manageable and kind of, typical synth sound. Now an interesting thing about the filter is, what happens when we keep on reducing it? [SOUND] You see that oscilloscope? It starts looking more and more like a sine wave. As we move those, as we remove those upper harmonics, we remove the upper harmonics in the spectrum display. Our oscilloscope starts looking more and more like a sine wave as I bring that filter lower and lower. It's pretty interesting how the relationship between wave shape and spectrum really works out. If I try the same thing with square wave we get a similar thing happening. I'll start with the cut off frequency high. [SOUND] It's very bright. I'll start bringing it down. [SOUND] And we get more and more toward a sine wave. Once we have just that single fundamental frequency going. We get more and more like a sine wave. Now, one thing we mentioned earlier was that, if we have high resonance, we can notice the individual peaks of the individual harmonics as we sweep the cutoff frequency. And this is a sound you'll start to notice in synth sounds in general. Let's try it out. [SOUND]. I'll increase resonance. And then I'll start sweeping the cutoff frequency. [SOUND] See, it gets so loud when I get over those harmonics, I start getting distortion. There's another harmonic. Another one. And there you go. So with high resonance we get a peak right at the cutoff frequency, and as you sweep that it will emphasize those individual harmonics drastically. We can also notice this as a steep peak if you're using noise. [NOISE] Since there's no individual harmonics on noise and there's just energy everywhere we hear it as a smooth sweeping across the frequency range. Sounds a lot like a whistle, doesn't it? You want to start noticing what high resonance sounds like. and when you're creating synth patches, don't feel like you always need resonance. Many synth patches don't have any at all, and it can sound great. Resonance is useful when you're trying to emphasize the sound of the filter. To make the filter an obvious thing in your patch. When you want the filter just to remove unwanted harmonics or unwanted frequencies. Then use no resonance. We also have the other kind of standard synth shapes. So I was using low pass which is the standard, but we also have high pass which emphasizes the high end and it sweeps. [SOUND] Sounds good. It just ends up with a high sizzle as the frequency range gets high. We, we also have band pass which is great at emulating for frequencies of the voice particularly at higher resonances. [SOUND] And with maybe a square shape. [SOUND] We just get a little. And a peak in the range, and we kind of avoid that fundamental entirely. And then with the notch filter we can get nice kind of hollow regions in the frequency range. [NOISE] And that noise, that also becomes very obvious. [NOISE] So we get that phaser sound that we had when we were looking at effects. So those are the common things you'll have to adjust in a filter. I'd like you to go to your synthesizer and see if you can locate the filter section. It'll take some time to figure out how it works. I think the most important thing about learning synthesis is experimenting. That's why I'm keeping this week kind of short. I want you to spend the time experimenting with sounds, see what you can come up with. Can you take the techniques we're looking at here, and the concepts we're looking at here, and figure out how to apply them in your synthesizer? The other thing I'd like to point out is that manuals are excellent. And very often on a synthesizer, the manual of a synth will give you a wonderful rundown of synthesis in general, and specifically how to apply these exact techniques in your synthesizer.

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